1. Field of the Invention
This invention relates to polyurethane interlayers and to laminated glass articles prepared therefrom.
2. Description of the Prior Art
Safety glass is a well-known term for a glass sandwich composed of a plastic interlayer bonding together two glass sheets so that the breakage of the glass results in minimum dispersion of fragments of broken glass. Laminated glass is used widely in automobiles and must possess a number of properties, among which are: (1) a high impact energy absorption level so as to minimize concussive injury; (2) a shear and tear strength sufficient to prevent rupture from the broken glass; (3) sufficient adhesion to the glass in order to inhibit laceration and prevent broken pieces of glass from flying about and injurying anyone nearby; and (4) good optical transparency.
One difficulty encountered in present-day windshields is that upon breaking, sharp glass edges develop which can result in severe lacerative injuries. Bilayer windshields comprising an outer ply of glass and an inboard ply of plastic have been suggested for minimizing lacerative injury. However, since the plastic layer will be exposed to the atmosphere, it must have excellent weathering properties, retaining its optical clarity and energy-absorbing properties upon exposure to the ambient atmosphere.
Since about 1930, plasticized polyvinyl butyral has been the most widely used interlayer material. However, its use has been restricted to trilayer windshields in which the polyvinyl butyral interlayer is sandwiched between two outer plies of glass. Polyvinyl butyral is particularly sensitive to moisture and will haze upon prolonged exposure to the ambient atmosphere thus negating its use in bilayer windshield configurations. In addition, polyvinyl butyral has extremely poor energy-absorbing properties at low temperature, that is, about 0.degree. to 30.degree. F. (-18.degree. to -1.degree. C.).
There has been a trend in recent years to provide a substitute plastic interlayer material for polyvinyl butyral. U.S. Pat. Nos. 2,333,639; 2,864,780; 2,601,312; 3,226,354; 3,388,032; 3,522,142; 3,620,950 and Belgian Pat. No. 785,125 all disclose the use of various polyurethanes for use in fabricating motor vehicle safety glass. The polyurethane materials in general offer improved physical properties over polyvinyl butyral in that they have better impact resistance at both high and low temperatures. There are, however, a number of disadvantages associated with certain of the above-mentioned polyurethanes. A particular disadvantage is that many of the polyurethane formulations bloom or develop hazy surface appearance upon exposure of the interlayer to the ambient atmosphere. This bloom reduces the optical transparency and curtails the desirability of such materials for use in bilayer laminate configurations.
Also, many of the aforementioned polyurethanes have extremely short pot lives, because high mix temperatures must be used to keep the reactants in melt form and the reaction mixture homogeneous. Usually after the polyurethane-forming reactants are initially mixed together, they polymerize to a gel very quickly, thereby minimizing their use for forming interlayers in safety glass laminates by the casting and curing-in-place technique such as disclosed in U.S. Pat. No. 3,522,142 mentioned above.